Add path package

For testing purposes we need to get the long path of filesystem paths
on Windows machines. By default non-UTF8 syscalls will return the short path
(Ex:"C:\PROGRA~1" instead of "C:\Program Files"). This package exports
previously private functions used in the symlink package to determine the
long path of a file/directory.
Let's not expose the ugly version, only the nice string version.  I know this means symlink will have to do an extra conversion back to []uint16, but I'd rather that than have to clutter the interface with a version that no one outside of symlink will want to use.